1. Technical Field
The present invention relates to a method of manufacturing spark plugs attached to an internal combustion engine mounted on an automobile.
2. Related Art
In an internal combustion engine, spark plugs are mounted to start the engine. The conventional spark plug is provided with a pillar-like center electrode held in an insulation manner within a housing member and an earth electrode one end of which is joined to the housing. The earth electrode is bent at its intermediate predetermined portion so as to make its other end (free end) face a tip of the center electrode. On the other end of the earth electrode, there is formed a noble metal chip. Thus ending the earth electrode is made to form a predetermined-size space, called “spark gap,” between the noble metal chip and the tip of the center electrode.
How to manufacture the spark plug is proposed by, for example, Japanese Patent First Publication No. 2002-164149. This publication shows a technique of measuring various positions, such as the position of the tip of the center electrode, before bending the earth electrode. The measured results are then used to determine an amount of bending which allow the size of the spark gap to fall into a predetermined tolerance range assigned thereto.
However, in the case of the above conventional bending technique, it is difficult to control, with precision, the position of the other end of the earth electrode in a lateral direction perpendicular to an axial line passing through the center electrode. This means that the above simple bending technique is always involved in positional fluctuations of the tip end (the other end) of the earth electrode in the lateral direction. Such fluctuations are directly reflected in the position of the noble metal chip in the lateral direction.
As a result, in the spark plug in which the noble metal tip is bonded on the earth electrode, the coaxiality between the center electrode and the noble metal chip is decreased, which is one of the reasons that ignitionability of spark plugs lowers.